dm-raid
  -------

  The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
  It allows the MD RAID drivers to be accessed using a device-mapper
  interface.

  The target is named "raid" and it accepts the following parameters:
  
    <raid_type> <#raid_params> <raid_params> \
      <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
  
  <raid_type>:

    raid1		RAID1 mirroring

    raid4		RAID4 dedicated parity disk
    raid5_la	RAID5 left asymmetric
  		- rotating parity 0 with data continuation
    raid5_ra	RAID5 right asymmetric
  		- rotating parity N with data continuation
    raid5_ls	RAID5 left symmetric
  		- rotating parity 0 with data restart
    raid5_rs 	RAID5 right symmetric
  		- rotating parity N with data restart
    raid6_zr	RAID6 zero restart
  		- rotating parity zero (left-to-right) with data restart
    raid6_nr	RAID6 N restart
  		- rotating parity N (right-to-left) with data restart
    raid6_nc	RAID6 N continue
  		- rotating parity N (right-to-left) with data continuation
  
    Refererence: Chapter 4 of
    http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
  
  <#raid_params>: The number of parameters that follow.
  
  <raid_params> consists of
      Mandatory parameters:
          <chunk_size>: Chunk size in sectors.  This parameter is often known as
  		      "stripe size".  It is the only mandatory parameter and
  		      is placed first.
  
      followed by optional parameters (in any order):
  	[sync|nosync]   Force or prevent RAID initialization.
  
  	[rebuild <idx>]	Rebuild drive number idx (first drive is 0).
  
  	[daemon_sleep <ms>]
  		Interval between runs of the bitmap daemon that
  		clear bits.  A longer interval means less bitmap I/O but
  		resyncing after a failure is likely to take longer.
  
  	[min_recovery_rate <kB/sec/disk>]  Throttle RAID initialization
  	[max_recovery_rate <kB/sec/disk>]  Throttle RAID initialization

  	[write_mostly <idx>]		   Drive index is write-mostly

  	[max_write_behind <sectors>]       See '-write-behind=' (man mdadm)
  	[stripe_cache <sectors>]           Stripe cache size (higher RAIDs only)

  	[region_size <sectors>]
  		The region_size multiplied by the number of regions is the
  		logical size of the array.  The bitmap records the device
  		synchronisation state for each region.

  
  <#raid_devs>: The number of devices composing the array.
  	Each device consists of two entries.  The first is the device
  	containing the metadata (if any); the second is the one containing the

  	data.

  
  	If a drive has failed or is missing at creation time, a '-' can be
  	given for both the metadata and data drives for a given position.
  
  
  Example tables
  --------------

  # RAID4 - 4 data drives, 1 parity (no metadata devices)

  # No metadata devices specified to hold superblock/bitmap info
  # Chunk size of 1MiB
  # (Lines separated for easy reading)

  0 1960893648 raid \
          raid4 1 2048 \
          5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81

  # RAID4 - 4 data drives, 1 parity (with metadata devices)

  # Chunk size of 1MiB, force RAID initialization,
  #       min recovery rate at 20 kiB/sec/disk

  0 1960893648 raid \

          raid4 4 2048 sync min_recovery_rate 20 \
          5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82

  'dmsetup table' displays the table used to construct the mapping.

  The optional parameters are always printed in the order listed

  above with "sync" or "nosync" always output ahead of the other
  arguments, regardless of the order used when originally loading the table.

  Arguments that can be repeated are ordered by value.

  'dmsetup status' yields information on the state and health of the
  array.
  The output is as follows:

  1: <s> <l> raid \
  2:      <raid_type> <#devices> <1 health char for each dev> <resync_ratio>

  Line 1 is the standard output produced by device-mapper.
  Line 2 is produced by the raid target, and best explained by example:

          0 1960893648 raid raid4 5 AAAAA 2/490221568
  Here we can see the RAID type is raid4, there are 5 devices - all of
  which are 'A'live, and the array is 2/490221568 complete with recovery.

  Faulty or missing devices are marked 'D'.  Devices that are out-of-sync
  are marked 'a'.